Induction heating devices are well known in the field of metal melting, forging billets of metal with a view to hot machining them, metal or alloy working or smelting. Nevertheless, in known devices, the induction coil(s) are wound around the cavity receiving the metal and are usually cooled by a water-cooling circuit. There is a possible risk of leaks in the cooling circuit, which is totally prohibited when working with molten metals. Furthermore, the efficiency achieved with this configuration generally does not exceed 40 to 60%. This efficiency is proportional to the ratio of the inductor's surface area and the surface area of the stack. What is more, the magnetic field created by the induction coils is an open field. Consequently, the losses are significant and amount to around 1/3 of the total power applied.
In this field of application, the main technical constraints to be taken into account are as follows:
protecting people from electromagnetic fields, as laid down by French standards and European directives (CENELEC and DG5), PA1 efficiency, and PA1 safety (it is essential that any contact between the water and the molten metal be avoided).
Other induction heating devices have attempted to provide a solution to the first problem posed. Some devices are described in the publications DE-C-266 566, US-A-1 834 725 and BE-A-351 671 and comprise at least two yokes arranged around the cavity receiving the metal to be heated, which are L-shaped or C-shaped, so that the ends converge toward the inside of said cavity. Each yoke bears an electric coil creating a magnetic field which closes through said cavity. An improvement to this type of construction is described in the publication DE-C-277 870 which provides for three yokes, the coils of which are fed individually and phase-shifted in order to create a rotary field. In all these embodiments, all the magnetic fields are radial, which means that the lines of electric flux cross the cavity's axis and cross right through it axially. These magnetic fields create an induction current limited to the periphery of said cavity and generate an increase in the temperature of the metal in this zone with the remainder of the metal being heated by conduction. The efficiency of these various devices and even the one providing for a rotary field, remains very low, as the effective part of the field using for heating purposes is small.